Method for improving the dispersibility of solvent-based paints in aqueous systems

ABSTRACT

A method for improving the dispersibility of over-sprayed, solvent-based paints in aqueous systems which comprises adding an effective amount of a chelating agent to an aqueous system containing solvent-based paints, particularly in high-hardness waters.

BACKGROUND OF THE INVENTION

Automobile bodies and many industrial and consumer articles are spraypainted in enclosed areas called paint spray booths. The purposes ofthese booths are to contain any fumes or paint overspray, reduce therisk of contamination of the part being painted and protect the healthof the workers at the site. Smaller booths may employ filters to removethe paint overspray from the air but larger systems usually use arecirculating water system for this purpose. In operation, the units tobe painted generally pass through the work area of the booth. Air flowthrough the booth forces the paint overspray into intimate contact withthe recirculating water, thereby effectively scrubbing the paint solidsfrom the air into the water. This may occur by forcing the air through awater curtain, some type of water spray or a venturi scrubber.Irrespective of how it is done, the net effect is that oversprayed paintcontacts and is captured by the recirculating water of the spray booth.

The amount of oversprayed paint contacting recirculating water in apaint spray booth may change depending on a number of variables,including plant or process shutdowns, the size and shape of the objectbeing painted, the type of spray equipment used, the spraying and purgetechnique used, the water flow rate and the type of paint used. Eventhough improved coating methods have significantly reduced the amount ofpaint overspray, up to about 50% of the total paint sprayed may notreach the article being painted. As a result, significant concentrationsof paint can build up in the system. Since the majority of paints usedtoday must be baked at elevated temperatures in order to cure properly,this paint can remain tacky indefinitely. It may coalesce into a mass ofsticky material that can plug pipes, fans and recirculation pumps. Inthis way it can significantly reduce scrubbing efficiency in the booth.This can lead to an air imbalance which would not only result in a poorpaint finish but also allow hazardous paint emissions to be dischargedinto the air. These conditions may also result in serious safety hazardsto paint spray booth personnel. Also, federal regulations now limit theamount of volatile organic compounds (i.e., vocs) that can be releasedat a given plant site. Organic solvent diluents used in solvent-basedpaint are a major source of vocs.

Other problems may also develop. For example, the tacky paint depositsare excellent food sources for bacteria and fungus growths. These inturn can cause odor problems and spot corrosion in the system. The paintsolids left in the water remain sticky and can cause expensiveseparation and disposal problems.

These problems demonstrate the desirability of effectively treatingpaint spray booth water systems so as to minimize the deposition ofsticky, oversprayed paint on critical operating parts of the system; tomake the resulting sludge non-tacky and easy to remove; and to providewater of sufficient quality that it can be recirculated in the system.

As paint technology has advanced, so has the quality of chemicaltreatments used to detackify the paint and provide a manageable sludge.There are presently a large variety of programs available for thispurpose. For example, the use of water soluble polymers in conjunctionwith amphoteric metal salts which form insoluble metal hydroxides atpH's>7 represents one such treatment. The use of combinations of thistype are described in the following U.S. Pat. Nos. 3,861,887 to Forney;3,990,986 to Gabel et al; 4,002,490 to Michalski et al; 4,130,674 toRoberts et al; and 4,440,647 to Puchalski. Further, U.S. Pat. No.4,637,824 to Pominville discloses the use of silicates andpolydiallyldialkyl ammonium halides with amphoteric metal salts, andU.S. Pat. No. 4,853,132 to Merrell et al discloses the use ofprecipitates formed by the reaction of cationic polymers and salts ofinorganic anions to detackify solvent based paints. Bentonite clays,aluminum salts and zinc salts have also been used with cationicpolymers.

U.S. Pat. No. 4,656,059 to Mizuno et al relates to the use ofmelamine-aldehyde acid colloid solutions for treating paint in wet spraybooths and U.S. Pat. No. 4,629,149 to Leitz et al relates to the use ofurea or amino triazine-aldehyde condensation reaction products incombination with water swellable clays to treat paint spray boothwastes. EP 0293129 discloses the use of melamine formaldehyde-type acidcolloids, in conjunction with an alkalinity source, to treat oversprayedsolvent based paints in paint spray booths, and U.S. Pat. No. 4,935,149discloses the use of formaldehyde scavenged melamine-formaldehydepolymers to treat paint spray booth waters. Copending application U.S.Ser. No. 588,997 relates to the use of melamine formaldehyde acidcolloids in paint spray booths containing both water and solvent basedpaints.

None of these references, however, disclose or suggest the use of achelating or sequestering agent to adjust the hardness of therecirculating water of a paint spray booth. The instant inventor hasfound the addition of such compounds to substantially improve theperformance of paint spray booth treatment methods, particularly acidcolloid-based methods, by increasing the dispersibility of solvent bornepaints captured by paint spray booth waters.

SUMMARY OF THE INVENTION

More particularly, it has been discovered that chemical paint spraybooth treatments, and in particular acid colloid spray booth treatments,generally only detackify the surface of a solvent-based paint droplet inthe recirculating water of a paint spray booth. A substantial portion ofeach paint droplet therefore remains unaffected by the treatment.Anything that penetrates or ruptures the surface of a droplet istherefore capable of releasing sticky paint from the center. Assumingthat the thickness of the detackified layer is roughly constant, itbecomes apparent that the larger the size of the droplet, the easier itis to rupture its surface and the less effective the overalldetackification program becomes. Conversely, as paint solids are moreeffectively dispersed and the average size of the paint dropletsdecreases, it becomes more difficult to rupture their surfaces, whichmeans that the detackification program becomes more effective.Therefore, any additives which are capable of improving the dispersionof paint particles in the recirculating water phase will also improvethe effectiveness of the detackification program.

Many modern paint spray booths utilize high energy venturi scrubbers tofacilitate the transfer of oversprayed paint from the air phase to thewater phase. The high mix energy of these systems improves scrubbingefficiency and also provides improved contact between the paint anddetackification chemicals, which serves to mechanically disperse thepaint and prevent the droplets from reagglomerating. Unfortunately, notevery facility is equipped with these high energy systems. Even wherehigh efficiency scrubbers are in use, the advent of new, high solids,solvent-based paints lessens the effectiveness of the mechanicaldispersion that these systems provide.

Several attempts have been made to address these situations chemicallywith limited results. For example, previously cited U.S. Pat. No.4,656,059 suggests the use of surface active agents to improve paintdispersibility and EP 0293129 discloses the use of selected alkalinitysources to achieve improved dispersion. Both of these methods were atleast partially successful relative to the types of paint used at thetime of the respective inventions. Unfortunately, in situations wherethe recirculating water contains substantial hardness or when any of avariety of the new generation of high solids, solvent-based paints areused, these methods are largely ineffective in providing adequatedispersion to effectively detackify the paint.

In an effort to remedy this problem, it has been discovered that theaddition of at least one conventional chelant to recirculating paintspray booth water substantially improves the dispersibility ofsolvent-based paints therein, particularly high-solids solvent basedpaints, thereby improving the efficacy of paint spray boothdetackification methods. While lowering the hardness via addition of achelant is believed to be beneficial in any case, the improvementbecomes more apparent in the presence of hardness levels above about 100mg/L, and is particularly evident in the presence of high hardnesslevels, such as is found in the American mid-west where source waterhardness levels of in excess of 500 are common. Thus, in its broadestsense, the instant invention is directed to a method for improving theperformance of paint detackification treatment methods by increasing thedispersibility of solvent-based paints lacquers or enamels in paintspray booth recirculating water, particularly when the water containssubstantial hardness, by the addition of an effective amount of achelating agent to the recirculating water being treated. This novelmethod therefore comprises the step of adding an effective amount atleast one chelant to the recirculating water of the paint spray booth,thereby facilitating and/or improving the dispersion of thesolvent-based paint, lacquer or enamel captured therein and enhancingthe detackification performance of an existing paint spray boothtreatment method, particularly a method which relies upon an acidcolloid such as a melamine formaldehyde acid colloid as the detackifier.While it is believed that the instant chelating agents complex withhardness, which causes improved dispersion of solvent-based paints inpaint spray booth water, particularly when alkalinity is added incombination with an acid colloid, the inventor does not wish to be boundby a particular mechanism.

Aside from improving paint dispersion, the addition of at least onechelating agent tends to extend the range of water chemistry in whichsolvent-based paints can be effectively treated. This inventiontherefore makes existing treatment methods more versatile in dealingwith pH and alkalinity fluctuations common in operating systems, andwith the variations in hardness common in waters found in certain areas.In addition, paint spray booth systems with low mix energy which werepreviously deemed difficult or impossible to effectively treat can nowbe handled with a minimum of difficulty.

The term "solvent-based paints", as used herein, refers to all varietiesof water-insoluble coatings applied in spraying operations, includingbut not limited to oil base paints, enamels, urethanes and lacquers.These products, if untreated, readily adhere to the walls of spray boothsystems and to any other surfaces that they contact, such as thesurfaces of water distribution piping, spray nozzles, and the like. Asused herein, the terms "solvent-based paints" and "solvent-borne paints"are synonymous.

The term "high solids paint", as used herein, refers to paints in whichthe percent solids has been increased over that of paints commonly usedin the 1970,s primarily by increasing the percentage of resin binder inthe paint at the expense of solvent. Typically, these paints contain inexcess of about 50% solids (weight basis), though the % solids may varysignificantly from paint to paint.

A primary treatment objective relative to solvent-based paints concernsthe tacky or adhesive nature of the over-sprayed coating material. Dueto their hydrophobicity, solvent-based solids tend to coalesce andaccumulate on the walls, ceilings and floors of spray areas and to clogwater spray equipment, recirculating pumps and the like. Thus, theover-sprayed paint mist captured in the water system of a spray boothmust be detackified, or "killed," to prevent adherence to the walls,piping, etc. of the spray booth system. Paint that sticks to spray boothsurfaces usually cannot be easily removed from the equipment and tendsto build up over time, thereby hampering spray booth efficacy.

Acid colloids are widely used to detackify or kill oversprayedsolvent-based paints. This invention relates to a method for improvingthe effectiveness of paint spray booth treatments by adding an effectiveamount of a chelating agent to the recirculating water of a paint spraybooth treating oversprayed solvent-based paint. The chelating agentimproves the dispersibility of the paint particles, which improvesdetackification efficacy. The chelants also improve the range ofoperability of conventional acid colloid treatment programs relative topH, alkalinity and hardness.

As used herein, the term "chelating agent" refers to any compoundcapable of solubilizing or complexing metal ions, particularly calciumand magnesium. As such, this term, as used herein, is synonymous withthe terms "solubilizing agent", "sequestering agent" and "complexingagent" when such terms refer to the ability of a compound to solubilizeor complex with metal ions or the ability of a compound to lowerhardness in a given paint spray booth system.

As used herein, the term "acid colloid" includes, but is not limited to,any acidic colloidal suspension of a melamine, dycyandiamide or ureacondensation product, alone or in combination, with one or morealdehydes. For example, acid colloids prepared using melamine andformaldehyde are suitable for use in this invention. Other suitable acidcolloids include those formed by the addition of a highly chargedcationic material to a silicate or a colloidal silica. Such acidcolloids are well known in the art.

DETAILED DESCRIPTION OF THE INVENTION

Despite the previously mentioned prior art efforts, there remains aclear need in the art for a method to provide a consistently effectivemeans of dispersing all types of solvent-borne paints in order tomaximize the effectiveness of conventional detackification programs. Inaddition, the treatment ideally should allow the formation of a sludgethat is high in solids and of low volume and the treatment should beeasy to apply and control.

In one embodiment, the present invention is directed to a method forimproving the efficacy of a solvent-based paint detackification methodwhich comprises adding an effective amount of a chelating agent to watercontaining oversprayed solvent-based paint prior to detackification ofsaid oversprayed paint.

In another embodiment, the present invention relates to an improvedmethod for treating circulating paint spray booth water which containshardness and which contains or which will contain over-sprayedsolvent-based paint(s), wherein the improvement comprises treatinghardness present in said water via the addition of an effective amountof a chelating agent, thereby improving the dispersibility of saidsolvent-based paint in said water.

The present invention also relates to a method for improving thedispersibility of oversprayed, solvent-based paint in the circulatingwater of a paint spray booth comprising adding to said water aneffective amount, preferably at least about 0.1 mg/L, based on theweight of the water being treated, of a chelating agent to said water.

Further, in a method for treating the circulating water of a paint spraybooth system used to capture over-sprayed, solvent-borne paint, whereinsaid circulating water contains hardness and wherein said methodcomprises adding an acid colloid for the purpose of killing ordetackifying said over-sprayed, solvent-borne paint, alone or incombination with an alkalinity source, the instant invention relates tothe improvement which comprises adding an effective amount of achelating agent to said circulating water for the purpose of treatingsaid hardness and improving the dispersibility of said solvent-basedpaint, thereby improving the efficacy of said acid colloid relative tothe killing or detackification of said over-sprayed paint.

Still further, the instant invention is directed to a method forimproving the efficacy of a melamine-formaldehyde-based paint spraybooth treatment program wherein the circulating water of said paintspray booth is treated with a melamine formaldehyde-type polymer for thepurpose of detackifying over-sprayed, solvent-based paint, which methodcomprises adding an effective amount of a chelating agent to saidcirculating water for the purpose of improving the dispersibility ofsaid paint.

Additionally, the instant invention is directed to compositions whichcomprise a) the circulating water of a paint spray booth containingoversprayed solvent-based paint and hardness; and b) at least about 0.1mg/L, based on the weight of said water, of a chelating agent.Optionally, these compositions may further comprise an acid colloid suchas a melamine aldehyde acid colloid and an alkalinity agent.

In this invention, at least one chelating agent is added torecirculating paint spray booth water as a conditioner/softener. It isbelieved that the chelants complex, sequester and/or chelate hardnessions present in the spray booth water, thereby preventing such ions frominterfering with the dispersion of solvent-based paint(s) in the systembeing treated. For a given system, a chelating agent will theoreticallycomplex, sequester and/or chelate a stoichiometric amount of hardnesspresent in the system being treated. Therefore, improved efficacy occursto the extent that hardness interference is reduced. By tying-uphardness, systems thought to be untreatable can now be effectivelyhandled The consumption of treatment chemicals such as alkalinitysources, if used, and acid colloids also generally decreases.

An effective amount of chelant should be used. As used herein, the term"effective amount" refers to that amount of chelant which improves thedispersibility of paint in the system being treated to the desiredextent. Preferably, at least about 0.1 mg/L of a chelant is added, basedon the weight of water in the system being treated.

Alternatively, the chelant can be added on a molar basis relative tohardness, so as to complex with a substantial amount of the hardnesspresent. Preferably, a sufficient amount of a chelating agent is addedto maintain hardness in the circulating system being treated below about100 mg/L, as CaCO₃, more preferably to below about 40 mg/L.

While it is believed that virtually any chelating agent whichsubstantially lowers the hardness of paint spray booth circulatingwaters can be used, examples of classes of chelants which are believedto provide the desired performance properties include, but are notlimited to phosphonic acids, aminocarboxylic acids, hydroxycarboxylicacids, polyamines, aminoalcohols and polymeric chelants. Specificexamples include, but are not limited to ethylenediaminetetraacetic acid(EDTA) and salts and derivatives thereof, nitrilotriacetic acid (NTA)and salts and derivatives thereof, diethylenetriamine pentacetic acid(DTPA) and salts and derivatives thereof, N-hydroxyethylethylenediaminetriacetic acid (HEDTA) and salts and derivatives thereof,tetraborates and triethylamine diamine.

In addition, various polyphosphates can be used. These phosphatecompounds have the added benefit of providing excellent corrosionprotection. The preferred phosphates are "molecularly dehydratedphosphates", by which is meant any phosphate which can be derived from amonobasic or dibasic orthophosphate or from orthophosphoric acid, orfrom a mixture of any two of these, by elimination of water ofconstitution therefrom. Examples of such phosphates include alkalinemetal tripolyphosphates, pyrophosphates, and metaphosphates, which areoften designated as hexametaphosphates.

While it is believed that any molecularly dehydrated phosphate may beemployed, it is preferred to use those which have a molar ratio ofalkaline metal to phosphorous methoxide from about 0.9:1 to about 2:1,the later being the alkaline metal pyrophosphate. While it is preferredto use the metaphosphates, pyrophosphates, or tripolyphosphates ofsodium, because they are the least expensive and most readily available,it is also possible to use the molecularly dehydrated phosphates ofother metals such as potassium, lithium, cesium, or rhobidium or theammonium molecularly dehydrated phosphates, which in many instances areclassified as being alkaline metal phosphates, or the alkaline earthmetal molecularly dehydrated phosphates such as those having calcium,barium, or strontium, or mixtures of alkaline metal and alkaline earthmolecularly dehydrated phosphates.

The most preferred water soluble phosphates are sodium hexametaphosphate(SHMP), such as "Calgon" which is available from Calgon Corporation,Pittsburgh, Pa., and which may be described as 1.1 NaO₂ :1P₂ O₅ ;tetrasodium pyrophosphate (TSPP); and sodium tripolyphosphate (STP).

A preferred class of chelants for purposes of this invention includesEDTA and salts thereof, NTA and salts thereof and polyphosphates. Sodiumsalts are most preferred.

The chelant may be added in either solid or liquid form to the paintspray booth recirculating water before the introduction of oversprayedsolvent-based paint. The chelant can be admitted to the paint spraybooth system at any convenient location via any conventional method.Typical points of addition include sumps, upstream from the pumps or atany advantageous position along the circulating water system.

The chelant may be slug fed or continuously fed to the paint spray boothwater system.

The effectiveness of a candidate chelant in dispersing solvent-basedpaint can be determined by a simple procedure. The procedure entails theuse of a number of containers to which water (preferably system water)and optionally a known concentration of a standard alkalinity sourcehave been added. Hardness of the samples is determined by conventionalmeans and a measured amount of the chelant to be evaluated is added.After an appropriate equilibration period, the hardness of the sample isagain measured to determine chelating capacity.

Typically, exceptional paint dispersion is obtained at a hardness ofless than about 100 mg/L, preferably 40 mg/L as CaCO₃. The effectivenessof the chelating agent can be observed visually by adding a detackifiersuch as a melamine formaldehyde acid colloid to the container whilestirring followed by an equivalent amount of the paint to be evaluated.After 30 seconds of vigorous stirring, the contents of the container aretransferred to a transparent beaker and observed. A well dispersed paintwill provide a uniform color to the water with no individual paintglobules visible in the beaker. In addition, no paint globules will befound adhering to the walls of the original container. Any chelatingagent which provides improved dispersion can be used in the instantmethod.

While the use of chelants is believed to improve the efficacy ofvirtually any solvent-based paint detackification method, they areparticularly effective relative to acid colloid-based treatment programs

Preferred acid colloids are polymers prepared from (a) a triazine suchas melamine or a substituted melamine, dicyandiamide urea orcombinations thereof; and (b) at least one compound described by thefollowing formula: ##STR1## wherein R₁ and R₂, which may be the same ordifferent, are selected from the group consisting of H and straight orbranched C₁₋₄ alkyl groups. The preferred compounds of (b) compoundscomprise aldehydes, with methanal (formaldehyde), ethanal and propanalbeing especially preferred; the most preferred aldehyde is formaldehyde.Such polymers may also include additional moieties, such asdicyandiamide or urea. However, nothing herein should be construed aslimiting the term "acid colloid" to polymers which contain only (a) and(b). For example, the acid colloids counterplated herein include theseformed by the addition of highly charged cationic material to silicateor a colloidal silica.

The preferred acid colloid polymers of the instant invention may beprepared by reacting the desired monomers according to standardcondensation polymerization procedures. See, for example, U.S. Pat. No.4,656,059. In general, such polymers are melamine/aldehyde condensateswherein the melamine mer units may be replaced by other NH₂ -reactivemonomers, but wherein melamine is the dominant NH₂ source in thecondensation reaction.

The mole ratio of component (a) to component (b) should generally rangefrom about 1:1 to about 1:6, with the preferred ratio being from about1:1 to about 1:3. The most preferred mole ratio is about 1 mole ofmelamine or a substituted melamine to about 2 to 2.5 moles of analdehyde. Thus, the most preferred polymer is prepared by reactingmelamine and formaldehyde with the mole ratio of melamine toformaldehyde being about 1:2 to about 1:2.5. Other compounds may also bereacted with the melamine and formaldehyde components to yield melaminepolymers or melamine-aldehyde-type polymers which are, for example,terpolymers. Compounds containing NH₂ groups are preferred comonomers.

The instant melamine polymers are insoluble in water. They are thereforebest utilized in acid solutions wherein the melamine polymer isstabilized in a fine colloidal state of suspension. Calgon's product CA289, which has a pH of about 1.6 to about 2.1, is an example of thepreferred form. This product contains about 8% activemelamine-formaldehyde polymer in an acid aqueous solution. Any acid canbe used to prepare the melamine aldehyde acid suspension, althoughhydrochloric acid is preferred. Also, other stabilizing agents, such aslow molecular weight alcohols and glycols, can be used.

The percent, by weight, of active melamine polymer in a stabilized(acidic) suspension or solution should range from about 0.1% to about20%, preferably 1% to about 15%, and most preferably about 4% to about12%, due to cost and product stability considerations. The pH should besufficiently low to keep the melamine formaldehyde-type polymer in afine colloidal suspension.

The molecular weight of the melamine formaldehyde-type polymer is notcritical. However., the preferred molecular weight generally ranges fromabout 500 to about 50,000, and the most preferred molecular weightranges from about 500 to about 5,000. As noted above, suitable melamineformaldehyde-type polymers are commercially available from CalgonCorporation, under the tradenames CA-289, WR-2511 and PK-9511. Theseproducts have molecular weights of about 2,200.

An effective amount of the acid colloid polymer should be added to ormaintained in the water being treated. Relative to the acid colloid, theterm "effective amount" refers to that amount of melamine polymer whichachieves the desired detackification of solvent-borne paint in thesystem being treated.

The melamine polymer can be applied intermittently or continuously tothe water system being treated at a preferred dosage of from betweenabout 0.001 to about 1.0 part melamine polymer (active basis) per partpaint, most preferably between about 0.01 and about 0.5 part activemelamine polymer per part paint. Since spraying is generally continuous,continuous addition of the melamine polymer is preferred. Forsolvent-borne paints, the melamine polymer must be added prior tocontact between live paint and recirculating paint spray booth water.The polymer can be added at any convenient location, but is preferablyadded so as to allow the maximum concentration of the melamine polymerto contact the over-sprayed paints. For combined water and solvent-bornesystems, it is desirable to add an effective portion of the melaminepolymer after contact between live paint and the circulating water.

In a typical paint spray booth operation, the return water from thebooth generally encounters extreme turbulence. This turbulence improvesthe efficacy of the treatment by promoting intimate contact between thepaint and the melamine polymer. The chelant is preferably added via anysuitable means to an area of high energy prior to the booth.

In a melamine aldehyde acid colloid-based treatment method, before theover-sprayed, solvent-based paint contacts paint spray booth water, thealkalinity of the water in the paint spray booth system is preferablyadjusted to maintain a minimum alkalinity of about 50 ppm (as CaCO₃) toa maximum alkalinity of about 2000 ppm. Preferably, the alkalinityshould be maintained between about 100 and about 1000 ppm (as CaCO₃). Ifsufficient alkalinity is not present (i.e., below about 50 ppm), theacid colloid treatment is generally not as effective, resulting in poorcollection efficiency and poor detackification of the solvent-basedpaint. Additionally, the pH of the water being treated should bemaintained between about 6.0 and about 9.0, preferably between about 6.5and 8.0. It is believed that the type of acid or base used to adjust andmaintain pH is not critical and that any acid or base can therefore beused. Acids and bases such as sulfuric acid and caustic, respectively,are generally used to adjust pH, if pH adjustment is necessary. A pH ofat least about 6.0 is desirable in order to activate the melaminepolymer. As the pH is lowered below about 6.0, corrosion in the systemgenerally increases. On the other hand, a pH of greater than about 9.0generally is expensive to maintain and facilitates foam generation andhinders solids capture.

Any alkalinity source can be used. For example, alkalinity can be addedin the form of alkali metal carbonates or alkali metal bicarbonates suchas sodium carbonate or sodium bicarbonate, or as NaOH, KOH, silicatesand/or blends thereof. The, preferred alkalinity sources are sodiumcarbonate or sodium bicarbonate. These alkalinity sources are generallyinexpensive and safe to use.

The alkalinity should be maintained so that the circulating water whichcontacts the over-sprayed paint or paints being treated is in thedesignated alkalinity range. Any method of adding the source ofalkalinity can be used, including batch or continuous addition, thoughcontinuous addition is preferred.

In acid colloid treatment methods, an effective amount of a polymericflocculant is generally added to

the paint spray booth water system. Relative to the flocculant, the term"effective amount" refers to that amount of flocculant necessary topromote the formation of a buoyant floc structure by binding theconditioned paint particles and incorporating air into the flocstructure. The resulting floating floc facilitates the removal of paintsolids from the circulating water system. The paint sludge generatedfrom co-addition of a polymeric flocculant and an acid colloid shouldprovide desirable handling and dewatering properties.

The type of polymeric flocculant used and its molecular weight are notbelieved to be critical to the detackification process. Suitableflocculants include cationic polymers, nonionic polymers, anionicpolymers, amphoteric polymers, or mixtures thereof.

Examples of acceptable flocculants include long chain polyacrylamidesand long chain polymethacrylamides; the preferred flocculants arenonionic or slightly anionic polyacrylamides having a weight averagemolecular weight ranging from about 6×10⁶ to about 20×10⁶.

Typical cationic polyelectrolytes which may be used as flocculants inthe instant invention include but are not limited to polyamines,polyphosphonium compounds, polysulfonium compounds, quaternary ammoniumcompounds, polymers of methacryloyloxethyl trimethyl ammonium methylsulfate (METAMS), polymers of methacrylamido propyl trimethyl ammoniumchloride (MAPTAC), polymers of acryloyloxyethyl trimethyl ammoniumchloride (AETAC), polymers of methacryloyloxyethyl trimethyl ammoniumchloride (METAC) and polymers prepared from combinations of METAMS,MAPTAC, AETAC and/or METAC with acrylamide and/or methyacrylamide.Representative of quaternary ammonium compounds are diethyl diallylammonium and dimethyl diallyl ammonium polymers and salts thereof.

The preferred cationic flocculants are quaternary ammonium polymers suchas polydimethyl diallyl ammonium chloride (polyDMDAAC), poly dimethyldiallyl ammonium bromide (polyDMDAAB), poly diethyl diallyl ammoniumchloride (polyDEDAAB), or any of the same copolymerized with acrylamideor methacrylamide. The preferred molecular weights for the quaternaryammonium polymers are in excess of about 2,000,000.

The most preferred cationic flocculant is a polymer comprising dimethyldiallyl ammonium chloride and acrylamide, or a homologue thereof, havinga weight average molecular weight in excess of about 4,000,000. Theratio of the nonionic moiety (for example, acrylamide or methacrylamide)to the cationic moiety should be greater than about 1:1, on an activeweight basis.

Other preferred polymers comprise: a) acrylamide or methacrylamide andb) METAMS, METAC, MAPTAC or AETAC, wherein the monomer ratio of a):b),on an active weight basis, is greater than about 1:1.

The preferred anionic polyelectrolytes possess a low charge (i.e., lessthan 10%) and are selected from the group consisting of polyacrylicacids and salts thereof, particularly sodium salts thereof, having amolecular weight ranging from about 2×10⁶ to about 20×10⁶, and polymerscomprising a) acrylic acid or methacrylic acid and b)2-acrylamido-2-methylpropylsulfonic acid and/or2-methacrylamido-2-methylpropylsulfonic acid, wherein the weight ratioof a):b) ranges from about 1:99 to about 99:1, preferably 10:90 to 90"10and most preferably 75:25, and wherein the molecular weight ranges fromabout 2×10⁶ to about 20×10⁶.

High molecular weight polyampholytes can be used in the instant method.Representative examples of suitable polyampholytes include polymerscomprising a) acrylic acid, methacrylic acid,2-acrylamido-2-methylpropylsulfonic acid or2-methacrylamido-2-methylpropylsulfonic acid, alone or in combination,and b) dimethyl diallyl ammonium chloride, dimethyl diallyl ammoniumbromide, diethyl diallyl ammonium chloride or diethyl diallyl ammoniumbromide, alone or in combination, wherein the weight ratio of componenta) to component b) ranges from about 90:10 to about 10:90 and whereinthe polyampholyte has a molecular weight in excess of about 2×10⁶. Thepolyampholytes may also contain nonionic moieties such as acrylamide ormethacrylamide.

Blends of the above listed nonionic, cationic, anionic and polyampholyteflocculants can be used, alone or in combination with amphoteric metalsalts.

An effective amount of the polymeric flocculant should be added. Theeffective amount for a given system generally depends upon the quantityof melamine aldehyde-type-polymer present in the system being treated.Preferably, the effective flocculant dosage will range from about 0.01to about 150 parts (active basis) of the polymeric flocculant per partmelamine aldehyde-type polymer (active basis). In this range, the use ofan appropriate polymeric flocculant forms a well conditioned,easily-captured floc, and in addition it may reduce or totally eliminatefoam formation in the system by removing colloidal particulates presentin the water.

After contacting the acid colloid polymer-treated paint solids with atleast one polymer flocculant, the sludge is removed from the water. Thisremoval may be facilitated by any means known in the art, including butnot limited to air flotation and filtration.

EXAMPLES

The following examples further demonstrate the present invention. Theyshould not be construed as limiting the scope of the invention in anyway.

EXAMPLE 1-21

A standard jar test procedure was used to determine the efficacy ofvarious chelants on a given system.

The general procedure involved adding 200 mls of system makeup water toa 500 ml glass jar. The initial hardness of the water was determined anda measured amount of chelant was added, based on the initial hardness.The combination was mixed for one minute at high speed on a magneticstirrer to dissolve the chelant and to allow for complexation withhardness. At the end of this time the hardness of the water was againmeasured.

One half (0.5) ml of PaintKill® 9511 (a melamine aldehyde acid colloiddetackifier commercially available from Calgon Corporation) was addedwith stirring followed immediately by one half (0.5) ml of the paint tobe evaluated. This composition was then mixed at high speed for 30seconds using a magnetic stirrer, after which time the contents werepoured into a 200 ml clear container. Effectiveness of chelant relativeto dispersion is readily discernible to the naked eye. A well dispersedpaint will produce a uniform, fine grained floc the same color as thepaint being treated with no globules of paint evident and no stickymaterial adhering to the walls of the jar while a poorly dispersed onewill show well defined globules randomly scattered in a translucentfloc. In addition, globules of material will be found adhering to thewalls of the jar.

A wooden tongue depressor can also be used to determine finerdifferences in the degree of dispersion (and consequentlydetackification performance) using the criteria defined in U.S. Pat. No.4,853,132 to Merrell et al. These guidelines are shown in Table I below.

To demonstrate the utility of the treatment a series of jar tests wereconducted on a mixed, high solids, solvent-based topcoat from Toyota,Georgetown, Ky. Results are illustrated in Tables II and III, below.

In Tables II and III:

Na₄ EDTA is sodium salt of ethylene diaminetetraacetic acid;

SHMP is sodium hexametaphosphate;

DTPA is diethylenetriaminepentaacetic acid;

HEDTA is hydroxyethylethlenediaminetriacetic acid;

TSPP is sodium tripolyphosphate; and

NTA is nitrilotriacetic acid.

                  TABLE I                                                         ______________________________________                                        DETACKIFICATION/DISPERSIONS GUIDELINES                                        Rating      Definition                                                        ______________________________________                                        Fail        Paint forms large tacky globules                                              and/or coating which adheres to the                                           exposed surfaces of the jar and                                               tongue depressor.                                                 Poor        Paint forms agglomerates which are                                            slightly tacky or smeary to touch,                                            or upon crushing. Paint sludge                                                costs the tongue depressor.                                       Fair        Paint forms granular flocs or                                                 globules which adhere to less than                                            10% of the exposed surface area of                                            the jar or depressor. Sludge may be                                           smeary, but not tacky.                                            Good        Paint forms particles or globules,                                            some of which appear as specks on                                             less than 1% of the exposed surface                                           area of the jar or depressor.                                                 Sludge is neither smeary nor tacky.                               Excellent   Paint forms particles which do not                                            adhere to the exposed surfaces of                                             the jar or depressor. The paint                                               sludge may float, sink, or be                                                 dispersed in the water.                                           ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        EFFECT OF VARIOUS CHELANTS                                                    ON DETACKIFICATION                                                                   Chelant                                                                Example                                                                              Active     Hardness  Dispersion/                                       #      (ppm)      as CaCO.sub.3                                                                           Detack  Comments                                  ______________________________________                                        1       --        188       Fail    Lg.                                                                           globules                                                                      coat jar                                                                      and                                                                           depressor                                 2      Na.sub.4 EDTA                                                                            32        Excellent                                                                             Completely                                       500                          dispersed                                 3      SHMP 500   30        Excellent                                                                             Completely                                                                    dispersed                                 4      Na.sub.4 P.sub.2 O.sub.7                                                                 38        Excellent                                                                             Completely                                       500                          dispersed                                 5      DTPA 500   30        Excellent                                                                             Completely                                                                    dispersed                                 6      HEDTA      36        Excellent                                                                             Completely                                       500                          dispersed                                 7      TSPP 500   40        Excellent                                                                             Completely                                                                    dispersed                                 8      NTA        --        Excellent                                                                             Completely                                       (excess)                     dispersed                                 9      Na.sub.2 B.sub.4 O.sub.7                                                                 --        Excellent                                                                             Completely                                       (excess)                     dispersed                                 10     Tri-       --        Excellent                                                                             Completely                                       ethyl-                       dispersed                                        amine                                                                         diamine                                                                       (excess)                                                               ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        EFFECT OF CHELANT CONCENTRATION                                               ON PAINT DETACKIFICATION                                                                    Hard-                                                                Alka.    ness                                                            Ex.  (1000    (as      Chelant Disp/                                          No.  ppm)     CaCO.sub.3)                                                                            (ppm)   Detack                                                                              Comments                                 ______________________________________                                        11   Na.sub.2 CO.sub.3                                                                      188      --      Poor  Visible                                                                       globules,                                                                     smeary to the                                                                 touch.                                   12   KOH      188      --      Fail  Lg. globules,                                                                 coat jar and                                                                  depressor                                13   Na.sub.2 Si.sub.3                                                                      188      --      Fail  Same as #12                              14   Na.sub.2 CO.sub.3                                                                      150      Na.sub.4 EDTA                                                                         Poor  Smaller globs                                                                 than #11. Small                                                               amount on                                                                     stirrer and                                                                   depressor.                               15   Na.sub.2 CO.sub.3                                                                      125      Na.sub.4 EDTA                                                                         Fair  More uniform                                                    200           dispersion but                                                                globs still                                                                   present. Some                                                                 on depressor.                            16   Na.sub.2 CO.sub.3                                                                       95      Na.sub.4 EDTA                                                                         Fair/ Some globules                                                   300     Good  present                                  17   Na.sub.2 CO.sub.3                                                                       62      Na.sub.4 EDTA                                                                         Good  V. sm. amount                                                   400           on depressor.                                                                 None on jar.                             18   Na.sub.2 CO.sub.3                                                                       32      Na.sub.4 EDTA                                                                         Excel-                                                                              Completely                                    500               500     lent  dispersed.                               19   Na.sub.2 CO.sub.3                                                                       48      Na.sub.4 EDTA                                                                         Good/ Few specks on                                 450               450     Excel-                                                                              depressor non                                                           lent  on jar. Lg.                                                                   globules, coat                                                                jar, depressor.                          20   Na.sub.2 CO.sub.3                                                                      190      --      Fail  Lg. globules,                                 500                             coat jar,                                     ppm                             depressor.                               21   Na.sub.2 CO.sub.3                                                                       45      Na.sub.4 EDTA                                                                         Excel-                                                                              Completely                                    500               500     lent  dispersed.                                    ppm                                                                      ______________________________________                                    

What is claimed is:
 1. A method for improving the dispersibility ofoversprayed, solvent-based paint in the circulating water of a paintspray treated with an acid colloid detackifier which is a melaminecondensation product with formaldehyde, wherein said paint is a highsolids paint containing in excess of about 50% solids, and said watercontains hardness, as calcium carbonate, in excess of about 100 mg/Lbased on the weight of said water, comprising adding to said water aneffective amount of a chelating agent selected from the group consistingof ethylenediaminetetraacetic acid or salts or derivatives thereof,diethylenetriaminepentaacetic acid or salts or derivatives thereof,N-hydroxyethylethylene diaminetriacetic acid or salts or derivativesthereof, nitrolotriacetic acid or salts or derivatives thereof,phosphates, triethylaminediamine, and tetraborates, to maintain saidhardness in said circulating water below about 100 mg/L.
 2. The methodof claim 1, wherein said chelating agent is selected from the groupconsisting of ethylenediaminetetraacetic acid or salts or derivativesthereof, diethyleneaminepentaacetic acid or salts or derivativesthereof, N-hydroxyethylethylenediaminetriacetic acid or salts orderivatives thereof, phosphates, triethylaminediamine and tetraborates.3. The method of claim 1, wherein at least about 0.1 mg/L of saidchelant is added.
 4. In a method for treating the circulating water of apaint spray booth system used to capture over-sprayed, solvent-bornepaint, wherein said circulating water contains hardness, as calciumcarbonate, in excess of about 100 mg/L based on the weight of saidwater, and said paint is a high solids paint containing in excess ofabout 50% solids, and wherein said method comprises adding a melamineformaldehyde acid colloid detackifier for the purpose of killing ordetackifying said over-sprayed, solvent-borne paint, alone or incombination with an alkalinity source, the improvement which comprisesadding an effective amount of a chelating agent to said circulatingwater for the purpose of treating said hardness and improving thedispersibility of said solvent-based paint, thereby improving theefficacy of said melamine formaldehyde acid colloid detackifier relativeto the killing or detackification of said over-sprayed paint, whereinsaid chelating agent is selected from the group consisting ofethylenediaminetetraacetic acid or salts or derivatives thereof,diethylenetriaminepentaacetic acid or salts or derivatives thereof,N-hydroxyethylethylene diaminetriacetic acid or salts or derivativesthereof, nitrilotriacetic acid or salts or derivatives thereof,phosphates, triethylaminediamine, and tetraborates, to maintain saidhardness in said circulating water below about 100 mg/L.
 5. The methodof claim 4, wherein said chelating agent is selected from the groupconsisting of ethylenediaminetetraacetic acid or salts or derivativesthereof, diethyleneaminepentaacetic acid or salts or derivativesthereof, N-hydroxyethylenethylenediaminetriacetic acid or salts orderivatives thereof, phosphates, triethylaminediamine and tetraborates.6. The method of claim 4, wherein at least about 0.1 mg/L of saidchelant is added.